We have induced specific immunological tolerance to dinitroflurobenze (DNFB) in B cell depleted mice. Tolerance which was induced with dinitrobenze sulfonate (DNBSO3) iv., was similar to that seen in normal mice. Thus, immune tolerance from iv. haptene does not require the participation of B cells or B cell products. The kinetics of desensitization are under study in this system. Allergic contact dermatitis can be induced in hamsters and passed with viable lymphoid cells. The histology of the sensitization site is consistent with allergic contact dermatitis. Studies of the regulation of the immune response to contact allergens in the hamster indicate that the hamster can be tolerized with allergen (DNBSO3) and that cyclophosphamide (Cy) immunopoteniation is successful in that animal. A suppressor T cell is implied. Previously, it was thought that the hamster uniquely lacked such suppressor T. cells. Studies of Cy immunopotentiation and of the heightened acquisition of delayed type sensitivity with complete Freund's adjuvant (CFA) indicate that contrary to reports in the literature, the phenomena are independent and, at least in part, additive. The responsible mechanisms are under analysis in a rat model of allergic contact dermatitis.